Method for the manufacture of a three-mirror optical system

ABSTRACT

The fabrication of an off axis three-mirror telescope on only two substrates allowing two of mirrors to be formed on a common substrate, such as by means of diamond turning. The common substrate configuration is made possible by utilizing a common vertex for both the primary and tertiary mirrors so that alignment subsequent to fabrication requires placement of only two elements. In addition, all three mirrors in this invention share a common optical axis enabling an extremely simple mounting and housing design. The present invention is applicable for use in electro-optical imaging sensors operating from visible wavelengths to long infrared wavelengths. A telescope is formed by the assembly of three off-axis mirrors, including a primary mirror, a secondary mirror and a tertiary mirror. In addition, the primary and tertiary mirrors also share a common vertex enabling the fabrication of two mirror surfaces on a common substrate with a diamond turning process, thus eliminating the need to align the primary and tertiary mirrors subsequent to fabrication. Consequently, system alignment consists of the placement of only two elements, namely the first element, the common substrate on which the primary and tertiary mirrors are formed and the second element which is the secondary mirror.

This is a division of application Ser. No. 08/048,575 filed Apr. 15,1993 now U.S. Pat. No. 5,414,555 which is a continuation of Ser. No.07/820,010 filed Jan. 13, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to three-mirror telescopes andmore specifically to a highly manufacturable three-mirror opticaltelescope in which the primary and tertiary mirrors share a commonvertex and all mirrors share a common axis. The primary-tertiary mirrorpair is fabricated on a common substrate with single point diamondturning, eliminating the need for aligning these two mirrors, resultingin great simplification of the fabrication and alignment processes. Theoptical system of the invention is particularly suitable for use as athree-mirror collimator or telescope in spacecraft applications.

2. Prior Art

Three-mirror optical systems useable as collimators and telescopes, forexample in spacecraft applications, are well known in the art. Oneexample of such a three-mirror optical system designed primarily as atelescope, is disclosed in U.S. Pat. No. 4,240,707 to Wetherell. Thispatent discloses radiation from an off-axis primary mirror reflectedback to an on-axis convex secondary mirror, which, in turn, reflects theradiation to an off-axis tertiary mirror. The radiation is thenreflected from the tertiary mirror to a focal plane, which is also offaxis. Another example of a three-mirror system is shown in U.S. Pat. No.4,265,510 to Cook, wherein a primary mirror, a secondary mirror and atertiary mirror form an anastigmatic relayed image-forming opticalsystem in which the image field is off the optical axis and the entrancepupil coincides with the primary mirror and an intermediate image isformed between the secondary and tertiary mirrors. Still anotherrelevant prior art patent, namely U.S. Pat. No. 4,737,021 to Korsch,discloses a three-mirror collimator or telescope having a focal planepositioned on a first side of the optical axis and which directs adiverging beam of radiation upon a concave tertiary mirror which is alsopositioned on the first side of the optical axis and which reflects thereceived radiation to an on-axis convex secondary mirror in a convergingpattern. A secondary mirror reflects the received radiation in a firstconverging, then diverging pattern upon a concave primary mirror whichis positioned on the second side of the optical axis. The primary mirrorreflects the received radiation as a collimated beam to the realentrance pupil which is located either on or near the optical axis, butwhich need not be precisely centered on the optical axis. The primarymirror reflects the radiation in a first converging, then divergingpattern upon the on axis secondary mirror which in turn reflects theradiation upon the off axis tertiary mirror. The tertiary mirror thenreflects and focuses the received radiation upon the focal plane.

One of the principal disadvantages of all of the known three-mirrorcollimator or telescope optical systems of the prior art is that theyare all configured in such a manner that requires each of the threemirrors to be separately fabricated and then assembled and alignedthereafter as a precise optical system. The alignment of a three-mirrorsystem in which all three of the mirror elements has been separatelyfabricated is a labor-intensive task which is more conducive to errorbecause of the need for precise placement of the three mirrors relativeto one another in the system. In most high performance three-mirrorsystems, the stringent misalignment tolerance between mirrors,particularly the primary-tertiary pair, renders it impractical tofabricate. Consequently, a method for fabricating such a three-mirrorsystem which reduces the complexity of the alignment and placement task,would be a highly desirable feature. Such a fabrication method isdisclosed herein and constitutes a principal feature of the presentinvention.

Other prior art patent disclosures deemed relevant to the presentinvention to varying degrees comprise the following:

    ______________________________________    2,729,143          White    3,801,180          MaGuire et al    4,239,342          Aurin et al    4,293,186          Offner    4,331,390          Shafer    4,469,414          Shafer    4,497,540          Breckinridge et al    4,733,955          Cook    4,737,021          Korsch    4,812,028          Matsumoto    5,009,494          Iossi et al    ______________________________________

U.S. Pat. No. 4,737,021 to Korsch is relevant to the present inventionin that it is directed to a three-mirror telescope arranged with acommon optical axis. Referring to FIG. 2, there is shown a three-mirrortelescope having an entrance pupil disposed on the optical axis, throughwhich a collimated beam is received by the primary mirror for reflectionto the on-axis secondary mirror, which in turn reflects the radiationupon the off-axis tertiary mirror for reflection to the focal plane.

U.S. Pat. No. 4,812,028 to Matsumoto is directed to a reflection typeprojection optical system for projecting micropattern images, such assemiconductor device patterns. Referring to FIG. 1, the reflectingsurfaces M1 and M3 are formed on a common substrate with all of themirrors in the system having a common optical axis. Although the twomirrors M1 and M3 have a common radius of curvature, the two mirrorshave a common vertex.

U.S. Pat. No. 4,293,186 to Offner is directed to an off-axis opticalsystem. Although directed to a projection system, the optical systemcomprises two spherical mirrors, a convex mirror and a concave mirror,wherein the system is arranged to provide three reflections. The mirrorsare arranged with their centers of curvature along the system axis withthe concave mirror having a portion above the axis providing one set ofreflections and another portion below the axis providing another set ofreflections.

U.S. Pat. No. 4,733,955 to Cook is directed to a reflective opticaltriplet for a telescope. Referring to FIGS. 3 and 4, there is shown, theplan and elevation views for the optical system wherein the radiationbeam enters through the real pupil for reflection from the primarymirror upon the secondary mirror and subsequently the tertiary mirror,for impinging upon a sensor. The optical elements are arranged to have acommon optical axis. The primary and tertiary mirrors are not disclosedas being formed on a common substrate.

U.S. Pat. No. 4,497,540 to Breckinridge et al is directed to an opticalsystem for viewing a remote surface. FIG. 7 illustrates an embodimentwherein the optical elements are arranged so as to have a common opticalaxis and a spherical mirror is defined by two reflecting surfaceportions. However, the surface portions each have the same radius ofcurvature.

SUMMARY OF THE INVENTION

The present invention comprises a fabrication method which overcomes theaforementioned disadvantages of the prior art. More specifically, theinvention described herein enables the fabrication of an off axisthree-mirror telescope on only two substrates. The present inventionovercomes the aforementioned labor intensive and sometimes impracticaltasks of aligning a three-mirror system by allowing two of mirrors to beformed on a common substrate, such as by means of diamond turning. Thecommon substrate configuration is made possible by utilizing a commonvertex for both the primary and tertiary mirrors so that alignmentsubsequent to fabrication requires placement of only two elements. Inaddition, all three mirrors in this invention share a common opticalaxis enabling an extremely simple mounting and housing design. Thepresent invention is applicable for use in electro-optical imagingsensors operating from visible wavelengths to long infrared wavelengths.A telescope is formed by the assembly of three off-axis mirrors,including a primary mirror, a secondary mirror and a tertiary mirror.All three mirrors share a common system optical axis. In addition, theprimary and tertiary mirrors also share a common vertex enabling thefabrication of two mirror surfaces on a common substrate with a diamondturning process, thus eliminating the need to align the primary andtertiary mirrors subsequent to fabrication. Consequently, systemalignment consists of the placement of only two elements, namely thefirst element, the common substrate on which the primary and tertiarymirrors are formed and the second element which is the secondary mirror.Because of the unique common substrate fabrication technique of thepresent invention, material cost is reduced by a significant amount,thus adding to the labor cost savings derived from the reduction in thecomplexity of the alignment task. In addition, there is improvedstructural stability due to the use of a common substrate for both theprimary and tertiary mirrors. There is also a reduction in mechanicaldeformation which is normally incurred in connecting mirrors to anoptical support structure. There is also a significant reduction inweight which is especially important for spaceborne applications. Also,because of the significant reduction in the likelihood of alignmenterrors between the primary and tertiary mirrors, there is a significantincrease in the tolerance for mirror fabrication errors in a constanttolerance budget for the entire optical system.

OBJECTS OF THE INVENTION

It is therefore a principal object of the present invention to provide amethod for manufacturing a three-mirror optical system which greatlysimplifies the fabrication and alignment processes required to realizesuch a system.

It is an additional object of the present invention to provide athree-mirror anastigmatic optical system for use as telescopes,collimators and the like, wherein the primary and tertiary mirrors sharea common vertex and all mirrors share a common axis and wherein theprimary-tertiary mirror pair is fabricated on a common substrate withsingle point diamond turning, eliminating the need for aligning thesetwo elements.

It is still an additional object of the present invention to provide animproved optical system and method of manufacture thereof for reducingfabrication time, alignment time and system weight while increasingmechanical stability.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and advantages of the present invention, aswell as additional objects and advantages thereof, will be more fullyunderstood hereinafter as a result of a detailed description of apreferred embodiment of the invention when taken in conjunction with thefollowing drawings in which:

FIG. 1 is a schematic optical ray path diagram of a three-mirror opticalsystem of the prior art;

FIG. 2 is a schematic optical ray path diagram of the present invention;

FIG. 3 is a schematic illustration used to explain the relativerelationships of the three mirrors of the present invention;

FIGS. 4 and 5 are elevational and side views respectively, of theprimary and tertiary mirrors of a three-mirror optical system fabricatedin accordance with the method of the present invention;

FIGS. 6 and 7 are schematic illustrations relating to the fabrication ofa primary and tertiary mirror for an optical system of the prior art andof the present invention respectively, for illustrating a significantdistinction therebetween; and

FIG. 8 is a schematic representation of a simplified alignment and testprocess made possible by the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference will first be made to FIG. 1 which illustrates a traditionaloff-axis three-mirror design commonly found in the prior art for use inthree-mirror telescopes. As shown therein, the three mirrors comprise aprimary mirror M1, a secondary mirror M2 and a tertiary mirror M3. Eachsuch mirror is structurally independent of the other two and each suchmirror has its own distinct axis which is separate from the axis of theother two mirrors. On the other hand, referring to FIG. 2, it will beseen that the three mirror configuration of the present invention, whichalso comprises primary mirror M1, secondary mirror M2 and tertiarymirror M3, is quite distinct from the prior art configuration of FIG. 1.More specifically, it will be seen in FIG. 2 that the three mirrors allshare a common axis and that furthermore, the primary mirror M1 and thetertiary mirror M3 share a common vertex. It will be seen hereinafterthat in addition to sharing a common vertex, primary mirror M1 andtertiary mirror M3 are fabricated on a common substrate, thereby fixingtheir relative positions to one another during fabrication and thusobviating the requirement for alignment upon assembly of the opticalsystem.

The meaning of a common axis and a shared vertex may be betterunderstood by reference to FIG. 3, wherein it will be seen that eachmirror M1, M2 and M3 may be characterized as a selected portion of acurved surface. The surfaces of M1 and M2 are spherical and the surfaceof M3 is an ellipsoid. Each such surface is associated with an axis, andas shown in FIG. 3, all three such surfaces are configured to have acommon unitary axis. Furthermore, the spherical surface of M1 and theellipsoid surface of M3 are positioned to share a common vertex as shownin the right-hand portion of FIG. 3. An additional feature of thepresent invention, the most significant one thereof from the standpointof manufacturability, is the shared common substrate of both M1 and M3,as seen best in FIGS. 4 and 5. FIGS. 4 and 5 are the front view and sideview respectively, of a primary and tertiary mirror of the presentinvention fabricated on a common substrate and representing an actualreduction to practice of the invention herein disclosed. In theconfiguration illustrated in FIGS. 4 and 5, the primary mirror M1 issubstantially circular in shape, having a vertically projected diameterof 6.25 inches, a vertex radius of 20 inches and a conic constant of-0.771. The tertiary mirror M3 is substantially rectangular, but havingrounded corners with a radius of about 0.75 inches, a vertical length of4.245 inches and a horizontal width of 5.186 inches. The distancebetween the center lines of the primary mirror M1 and the tertiarymirror M3 is 6.726 inches and the common optical axis line is 0.691inches above the center line of the tertiary mirror M3. Tertiary mirrorM3 has a vertex radius of 8.456 inches and a conic constant of -0.124.The substrate material may, by way of example, be aluminum, beryllium,silicon carbide or SXA.

The method of the present invention is preferably carried out usingdiamond turning on a lathe, the position of the diamond tip of thecutting tool being controlled in an axial direction as the cutting toolis moved radially, relative to the axis of the rotating substrate. Inorder to fabricate the primary and tertiary mirrors on a commonsubstrate in accordance with the present invention, the physical spacingbetween the two mirrors must be such as to avoid a machininginterference region so that the diamond cutting tool can first becontrolled to fabricate the surface curvature of one of the two mirrorsand then be independently controlled to fabricate the surface curvatureof the other of the two mirrors. As previously noted, the closest priorart known to the applicant is disclosed by Korsch in U.S. Pat. No.4,737,021. The Korsch patent discloses a design having two distinctmirrors sharing a common vertex and a common axis. However, theplacement of M1 and M3 in Korsch's disclosure does not meet thenecessary condition for fabricational clearance between M1 and M3 toprovide the opportunity to fabricate the two mirrors on a commonsubstrate using diamond turning. This deficiency in the prior art isillustrated in FIG. 6. FIG. 6 illustrates the relative size andplacement of M1 and M3 as disclosed by Korsch. Three circles are showntherein. The circle having the smallest diameter is the inner circle ofthe M1 or primary mirror fabrication annulus. It represents the positionof the diamond turning tool as it begins its radial motion in order tocut the surface geometry of M1, the primary mirror. The second largestcircle shown in FIG. 6 is the M3 fabrication circle, that is thecircular position of the diamond cutting tool upon completion of thesurface fabrication of the tertiary mirror M3. The largest circle, theoutermost circle of FIG. 6, represents the maximum displacement of thediamond cutting tool from the center or axis of the substrate uponcompletion of the surface fabrication of primary mirror M1. As seen inFIG. 6, there is a significant amount of overlap in the relativepositions of the diamond cutting tool for fabrication of M1 and M3. Morespecifically, because the inner circle of M1 fabrication has a smallerradius than the outer circle of M3 fabrication, the cross-hatched regionshown in FIG. 6 represents the machining interface region. Clearly, inorder to achieve diamond turning surface fabrication of two distinctmirrors on a common substrate, one cannot have such a machininginterference region. Consequently, those having skill in the art towhich the present invention pertains will now understand that thedisclosure of the prior art Korsch patent is readily distinguished fromthe present invention by the positioning of primary and tertiary mirrorsM1 and M3 respectively, which makes diamond turning surface fabricationimpossible on a common substrate.

On the other hand, referring now to FIG. 7, it will be seen that in thepresent invention there is no overlap between the machined regions ofthe primary and tertiary mirrors M1 and M3 respectively, and in fact asshown in FIG. 7, there is a gap or clearance annulus represented by theunshaded area in FIG. 7 between the outer fabrication circle of tertiarymirror M3 and the inner fabrication circle of primary mirror M1. Morespecifically, in FIG. 7, the cross-hatched area represents the machinedregion of the tertiary mirror M3 and the shaded area represents themachined region of the primary mirror M1. The machining of M3 isaccomplished by moving the diamond turning tool radially from the centervertex point at the center of the circles of FIG. 7, outward in a radialdirection. The same is true for the fabrication of M1. However, theradial travel of the cutting tool for fabrication of M3 is entirelydistinct from and non-overlapping with the radial travel of the cuttingtool for fabrication of M1.

As previously indicated above, one of the most significant advantages ofthe present invention is the reduction in the complexity of the opticalalignment process that is achieved by virtue of the common substrate andfixed relative positions of the primary and tertiary mirrors M1 and M3.In this regard, FIG. 8 illustrates test set-ups for initial alignmentand subsequent interferometer analysis of a three-mirror optical systemusing the present invention. For purposes of illustration, the primarymirror M1 and the tertiary mirror M3 are shown in FIG. 8 on a commonsubstrate that is cut down in size compared to the substrate shownpreviously in FIGS. 4 and 5. Initially, alignment of M2 relative to M1and M3 is achieved without the laser interferometer and without thelarge reference surface, both of which are shown in FIG. 8. The backside of the secondary mirror M2 is provided with a reference mirrorsurface which has an alignment mark thereon. Similarly, M3 is alsoprovided with an alignment mark at the intersection of the common axiswith M3. The distance between M2 and the substrate upon which M1 and M3are located, is determined by a housing based upon precise calculationand design. On the other hand, the position of M2 in and out of FIG. 8and vertically along FIG. 8 is determined by the alignment scope basedupon the coincidence of the mark on the back reference mirror of M2 andthe mark on M3 as viewed through the alignment scope. The principalsignificance of FIG. 8 is effectively what it does not show. Morespecifically, it does not show any need to reposition M1 relative to M3,which as previously noted are fixed relative to one another during thefabrication process because of the diamond turning of both mirrors on acommon substrate. This results in a significant reduction in the numberof variables that must be dealt with during the alignment process. Asthose having skill in the art to which the present invention pertainswill readily perceive, if it were necessary, as it is in the prior art,to reposition M1 and M3 relative to one another, each such repositioningwould require realignment of M2 relative to M1 and M3. The commensurateincrease in the number of variables would grossly complicate thealignment process, making alignment more of an emperical process withmore than one solution, some of which would not be as optimal asdesired. On the other hand, as shown in FIG. 8, with M1 and M3 fixedrelative to one another, and fixed also relative to the housing by meansof the substrate upon which they are fabricated, the only variables arethe relative position of M2 in three linear directions and in itsangular orientation relative to the substrate upon which M1 and M3 aremounted. Thus, the process of alignment is far simpler and lesstime-consuming and thus less costly as a result of the uniquefabrication process of the present invention.

After the alignment of M2 relative to the substrate of M1 and M3 iscompleted, the test process continues with the use of the laserinterferometer and the large reference surface which receives andreflects the laser image wavefront from M1. The laser interferometertest process is a standard one in the art and need not be describedherein in detail. Suffice it to say that the laser interferometer isoperated in conjunction with a lens for providing an image plane uponwhich there is generated an interference pattern, the character of whichdepends upon the proper alignment and orientation of each of the mirrorelements of the optical system shown in FIG. 8. Basically, theinterferometer measurement is a precise means for assessing the accuracyof the alignment of M2 as previously described. In the event that theinterferometer test shows that alignment is still not precisely correct,the system may still be aligned using the interferometer while makingvery precise minor adjustments with respect to M2 only. The fineadjustment of only M2 to optimize the alignment of the optical system ofFIG. 8 is made possible as a result of the common substratemanufacturing process of the present invention which obviates adjustmentof M1 and M3 in this final stage of alignment.

It will now be understood that what has been disclosed herein, comprisesa novel method of fabricating a three-mirror optical system such asthose used in spaceborne telescopes and comprising a primary mirror M1,a secondary mirror M2 and a tertiary mirror M3. In the inventiondescribed herein, all three such mirrors share a common axis and inaddition M1 and M3 share a common vertex, but most importantly, M1 andM3 share a common substrate and relative positions on such a substratewhich enable the fabrication of M1 and M3 from such a common substrateby diamond turning which obviates the independent and separatefabrication of M1 and M3. As a consequence thereof, there is asignificant savings in time and labor required to optically align thethree mirrors of such an optical system. There is also a significantreduction in material cost and improvement in structural stability dueto the use of a common material. There is also an advantageous obviationof mechanical deformation to connect M1 and M3 to an optical supportstructure, a reduction in fabrication time, improved mechanical andenvironmental stability, a reduction in weight which is especiallyimportant for spaceborne applications and easier compliance with overallsystem tolerance errors because of a reduction in the tolerance errorsattributable to the relative placement of M1 and M3. An embodiment ofthe novel method of the present invention may be characterized by thefollowing steps:

1. Placing a rough-machined and preshaped substrate of suitable mirrormaterial, such as aluminum, beryllium, silicon carbide or SXA into adiamond tool turning lathe;

2. Controlling the radial and axial position of the diamond turninglathe diamond cutting tool for surface fabrication of either M1 or M3;

3. Continuing diamond turning after completion of M1 or M3 fabricationfor surface fabricating the remaining mirror of M1 and M3; and

4. Removing the substrate from the lathe to form a single substratehaving M1 and M3 integrally fabricated thereon.

The rough-machining and preshaping indicated in step 1 is intended tominimized the diamond turning time by providing a near net shapesubstrate.

Those having skill in the art to which the present invention pertains,will now as a result of the applicants' teaching herein, perceivevarious modifications and additions which may be made to the invention.By way of example, although specific reference has been made to athree-mirror system for use in spaceborne applications and suitablematerials, shapes and dimensions have been disclosed for that purpose,it will now be understood that the fabrication method of the presentinvention may be suitable for the production of other plural opticalcomponents that are fixed relative to one another in an optical system.The steps of the novel method of the present invention mayadvantageously obviate subsequent alignment requirements for suchoptical components as hereinabove disclosed. Accordingly, all suchmodifications and additions are deemed to be within the scope of theinvention which is to be limited only by the claims appended hereto andtheir equivalents.

We claim:
 1. A method of fabricating the primary and tertiary mirrors ofa three-mirror optical system having a primary mirror, a second mirrorand a tertiary mirror, the primary and tertiary mirrors having differentsurface contours, the method comprising the steps of:a) selecting apre-shaped substrate of suitable material and defining a common vertexon said substrate for said primary and tertiary mirrors; b) turning saidsubstrate about an axis through said vertex and applying a cutting toolto a selected surface of said substrate; c) controlling the axialposition of said cutting tool while moving said cutting tool radiallyrelative to said axis to form a selected surface shape of said primarymirror; d) controlling the axial position of said cutting tool whilecontinuing to move said cutting tool radially relative to said axis toform a selected surface shape of said tertiary mirror; thereby leaving aunitary substrate having said primary and tertiary mirrors formedintegrally thereon.
 2. The method recited in claim 1 comprising theadditional step of creating an annular gap between said primary andtertiary mirrors along said selected surface of said substrate.
 3. Amethod of fabricating the primary and tertiary mirrors of a three-mirroroptical system having a primary mirror, a secondary mirror and atertiary mirror, the primary and tertiary mirrors having differentsurface contours, the method comprising the steps of:a) selecting apre-shaped substrate of suitable material and defining a common vertexon said substrate for said primary and tertiary mirrors; b) turning saidsubstrate about an axis through said vertex and applying a cutting toolto a selected surface of said substrate; c) controlling the axialposition of said cutting tool while moving said cutting tool radiallyrelative to said axis to form a selected surface shape of said tertiarymirror; d) controlling the axial position of said cutting tool whilecontinuing to move said cutting tool radially relative to said axis toform a selected surface shape of said primary mirror; thereby leaving aunitary substrate having said primary and tertiary mirrors formedintegrally thereon.
 4. The method recited in claim 3 comprising theadditional step of creating an annular gap between said primary andtertiary mirrors along said selected surface of said substrate.